Сraig. Dental Materials

Silica/Binder Ratio Investments usually contain 65% to 75% silica, 25% to 35% calcium sulfate hemihydrate, and about 2% to 3%of some additive chemicals to control the different physical properties and to color the investments. If the silica/stone ratio is increased, the hygroscopic expansion of the investment also increases, but the strength of the investment decreases.

Role of Water During the setting process, dental casting investments actually absorb water from their surroundings and expand. It also has been observed that, during setting, the more water an investment is permitted to take up from any source, the higher its hygroscopic expansion. The hygroscopic expansion of an investment during setting versus the amount of water added to its surface is shown in Fig. 13-12, curves A and B. As indicated, the more water added to the surface of the mixed investment, the higher the hygroscopic expansion, up to a point beyond which further additions of water do not create any additional expansion. This degree of expansion and its corresponding quantity of water is

High heat casting

.2.

.o.

.a .

Temperature ( O C )

called the criticalpoint. Note that for an investment to reach its maximum hygroscopic expansion, sufficient water should be available to it. If hygroscopically expanding investments are in contact with less water than they are able to absorb, they will not exhibit their maximum hygroscopic expansion.

HYGROSCOPIC-THERMAL GOLD CASTING INVESTMENT

There is one gold casting investment on the market that was designed for use with either hygroscopic or thermal type of casting techniques. Fig. 13-13 shows the high thermal expansion of this investment in the range between 482" C and 649" C. This expansion is high enough to use the investment with the thermal casting technique, without water immersion. However, when immersed in a water bath, the investment expands hygroscopically (Fig. 13-14). With the hygroscopic technique, the investment needs to be heated to only 482' C to provide the appropriate expansion.

Normal setting expansion

io 5 $0 5 io I&I I ~ O

Time in minutes

Fig. 13-13 Thermal expansion of mixed hygro- scopic-thermal gold casting investment.

(Courtesy Whip Mix Corp., 1992.)

Fig. 13-14 Setting and hygroscopic expansion of mixed hygroscopic-thermal sold casting investment.

(Courtesy Whip Mix Corp., 1992.)

INVESTMENT FOR CASTING HIGH-MELTING POINT ALLOYS

Most palladium and base metal alloys used for partial dentures and porcelain-fused-to-metal restorations have high melting temperatures. They should be cast at a mold temperature greater than 700" C. For this reason, calcium sulfate-bonded investments are usually not used for casting these alloys. Only one base metal alloy for dental applications possesses a low enough melting point to be cast into a mold at 700" C with a calcium sulfate binder. This alloy is an exception, because base metal alloys are usually cast into molds at 850 to 1100" C. To withstand these high temperatures, the molds require different types of binders, such as silicate and phosphate compounds. This type of investment usually has less than 20% binder, and the remainder of the investment is quartz or another form of silica.

Phosphate-Bonded Investment The most common type of investment for casting high-melting point alloys is the phosphatebonded investment. This type of investment consists of three different components. One component contains a water-soluble phosphate ion. The second component reacts with phosphate ions at room temperature. The third component is a refractory, such as silica. Different materials can be used in each component to develop different physical properties.

The binding system of a typical phosphatebonded investment undergoes an acid-base reaction between acid monoammonium phosphate (NH4H2P04)and basic magnesia (MgO). The soluble phosphate in water reacts with the sparingly soluble magnesia at its surface, forming a binding media with filler particles embedded in the matrix. The chemical reaction at room temperature can be expressed simply as follows:

The water produced by this reaction at room temperature lowers the viscosity of the mix as spatulation continues.

As the reaction takes place, colloidal particles are formed with a strong interaction among the particles. During setting and burnout, the sequence of chemical and thermal reactions causes various phase changes, providing roomtemperature strength (green strength) and hightemperature strength that enable the investment to withstand the impact of high-melting point alloys. Phases formed at high temperatures include Mg2P20, and subsequently Mg3(P04),. To produce higher expansion, a combination of different particle sizes of silica is used.

These investments can be mixed with water or with a special liquid supplied by the manufacturer. The special liquid is a form of silica sol in water. As shown in Fig. 13-15, phosphate-

Concentration of sol (%)

Fig. 13-15Effect of silica sol concentration on thermal expansion (solid lines) at 800'C and setting expansion (dotted lines) of two phosphate-bonded investments (A, thermal expansion type; B, hygroscopic expansion type).

(Adapted from Zarb GA, Bergman G, Clayton JA, MacKay HF, editors: Prosthodontic treatment for partiallyedentulous patients, St Louis, 1978, Mosby.)

bonded investments possess higher setting expansion when they are mixed with the silica sol than when mixed with water. With a mix containing silica sol, the investment mass is capable of expanding hygroscopically, whereas if the mix is only water, the hygroscopic expansion of such an investment is negligible. Not all phosphatebonded investments, however, can expand hygroscopically. Using silica sol instead of water with phosphate-bonded investment also increases its strength considerably. Fig. 13-16 shows thermal expansion curves of two commercial phosphate-bonded investments mixed according to the manufacturers' recommended liquid/powder ratio. Both the setting and thermal

Temperature ("C)

Fig. 13-16Thermal expansion curves of two phosphate-bonded investments mixed at recommended liquidlpowder ratios (A, thermal expansion type; B, hygroscopic expansion type).

(Adapted from Zarb GA, Bergman B, Clayton JA, MacKay HF, editors: Prosthodontic treatment for partially edentulous patients, St Louis, 1978, Mosby.)

expansions must be considered in selecting these investments.

ANSI/ADA Specification No. 42 (IS0 9694) for dental phosphate-bonded casting investments specifies two types of investments for alloys having a solidus temperature above 1080" C:

Type 1: For inlays, crowns, and other fixed restorations

Type 2: For partial dentures and other cast, removable restorations

The following properties are specified by the specification: fluidity, initial setting time, compressive strength, and linear thermal expansion. The values allowed for these properties are summarized in Table 13-12.

Silica-BondedInvestment Another type of binding material for investments used with casting high-melting point alloys is a silicabonding ingredient. This type of investment may derive its silica bond from ethyl silicate, an aqueous dispersion of colloidal silica, or from sodium silicate. One such investment consists of a silica refractory, which is bonded by the hydrolysis of ethyl silicate in the presence of hydrochloric acid. The product of the hydrolysis is a colloidal solution of silicic acid and ethyl alcohol, which can be written as

In practice, however, the reaction is more complicated, and instead of tetrasilicic acid, which is converted into SiO, . 2H20, a polymerized compound of silicon is formed with the following structure:

This material has an even higher silica content and better refractory properties than the SiO, 2H,O.

Ethyl silicate has the disadvantage of giving off flammable components during processing, and the method is expensive; thus other techniques and methods have been developed to reduce the use of this material. Sodium silicate and colloidal silica are more common binders of the silica type.

Today this investment is usually supplied with two bottles of special liquid, instead of water, with which the investment powder should be mixed. In one of the bottles the manufacturer usually supplies a properly diluted water-soluble silicate solution. The other bottle usually contains a properly diluted acid solution, such as a solution of hydrochloric acid. The contents of each bottle can be stored almost indefinitely. Before use, mix an equal volume from each bottle and allow the mixed liquids to stand for a prescribed time, according to the manufacturer's instnlctions, so hydrolysis can take place and freshly prepared silicic acid forms.

Proposed ANSI/ADA Specification No. 91 (IS0 11246) for ethyl silicate casting investments specifies setting time, compressive strength, and linear thermal expansion. The setting time must not differ by more than 30% from the time stated

by the manufacturer. The compressive strength at room temperature shall not be less tlian 1.5 MPa. The linear thermal expansion must not differ by more than 15% from the time stated by the manufacturer.

BRAZING INVESTMENT

When brazing (soldering) the parts of a restoration, such as clasps on a removable partial denture, the parts must be surrounded with a suitable ceramic or investment material before the heating operation. The assembled parts are ternporarily held together with sticky wax until they are surrounded with the appropriate investment material, after which the wax is softened and removed. The portion to be soldered is left exposed and free from investment to permit wax removal and effective heating before it is joined with solder.

ANSI/ADA Specification No. 93 (IS0 11244) for dental brazing investments defines two types of investment:

Type 1: Gypsum-bonded dental brazing investments

Type 2: Phosphate-bonded dental brazing investments

416 Chapter 13 GYPSUM PRODUCTS AND INVESTMENTS

The specification specifies quality, fluidity, setting time, compressive strength, linear thermal expansion, and linear setting expansion. The values allowed for these properties are summarized in Table 13-13.

The investment for soldering of low-melting- point alloys is similar to casting investments containing quartz and a calcium sulfate hemihydrate binder. For high-melting point alloys, a phosphate-bonded investment is used.

Soldering investments are designed to have lower setting and thermal expansions than casting investments, a feature that is desirable so the assembled parts do not shift in position during the setting and heating of the investment. Soldering investments are often made of ingredients that do not have as fine a particle size as the casting investment, because the smoothness of the mass is less important. Relatively little information is available in the dental literature on the properties of soldering investments.

INVESTMENT FOR

ALL-CERAMIC RESTORATIONS

Two types of investment materials have been developed recently for producing all-ceramic restorations. The first type is used for the cast

glass technique. This investment is provitled b y the manufacturer of the glass casting equipment and is composed of phosphate-bonded refractories. The second type of investment for making all-ceramic restorations is the refractory die type of material, which is used for all-ceramic veneers, inlays, and crowns. Refractory dies are made by pouring the investment into impressions. When the investment is set, the die is removed, and is heated to remove gases that may be detrimental to the ceramic (degassing). A refractory die spacer may be added to the surface. Next, porcelain or other ceramic powders are added to the die surface and fired. These materials must accurately reproduce the impression, remain undamaged during the porcelain firing, and have a thermal expansion compatible with that of the ceramic (otherwise the ceramic could crack during cooling). These materials are also phosphatebonded, and they generally contain fine-grained refractory fillers to allow accurate reproduction of detail. ANSI/ADA Specification No. 92 (IS0 11245) for phosphate-bonded refractory die tnaterials is under development.

I SELECTED PROBLEMS

Problem 1

High-strength dental stone dies sometimes fracture during separation from rubber impressions. How can this difficulty be minimized?

Solution a

The recommended wateripowder ratio should be used. The wateripowder ratio can vary from 0.19 to 0.24 for various Type 4 gypsum products. The water and powder should be dispensed accurately. Optimum strength is achieved only at the correct wateripowder ratio.

Solution b

Vacuum mixing of high-strength dental stone ensures maximum strength by minimizing porosity.

Solution c

The poured die should be allowed to set for at least 20 minutes or until final set before it is removed from the impression.

Solution d

Increasing the thickness of stiffer impression materials (polyether) increases the ease of removal of the die.

Problem 2

The occlusal surfaces of teeth of a Type 3 gypsum model poured from an alginate impression were chalky and friable. What may have happened, and how can this problem be solved?

Solution

Excess water in the depressions of an alginate impression (or any impression) from rinsing will increase the water/powder ratio of the

dental stone. Blood and saliva remaining on the impression retard the setting of the dental stone. Both conditions can cause the dental stone to be chalky and friable. Caref~~llyrinse and remove excess water from an impression before pouring it in dental stone.

Problem 3

The surface of a high-strength dental stone die was abraded during preparation of the wax pattern. Can Type 4 gypsum be treated to create a more abrasion-resistant surface that is less susceptible to damage during construction of the pattern and finishing of the casting?

Solution

Apparently not, because the available hardening solutions and various impregnation techniques have little effect on the abrasion resistance of Type 4 gypsum. Hardening solutions do result in a slightly higher setting expansion of high-strength dental stone dies.

Problem 4

A dental stone master cast for a complete denture was placed in a bowl of water before it was mounted on an articulator. Inadvertently, the cast was left in the water overnight. After the cast was mounted and dried, an unusually rough surface appeared. What happened?

Solution

Dental stone is slightly soluble in water. Leaving the cast in water for an extended time can dissolve some of the surface, roughening the cast. This roughness is transferred to the surface of the denture. If the cast must be stored in water, use a saturated calcium sulfate dihydrate solution (slurry water).

418 Chapter 13 GYPSUMPRODUCTSAND INVESTMENTS

Problem 5

The boxing edge of a dental stone master cast was trimmed on a model trimmer several days after it had been poured. Trimming was much more difficult than when done soon after the dental stone had set. Why was the cast more difficult to trim, and how can this difficulty be corrected?

Solution

As dental stone dries over a period of several days, its compressive strength increases to about twice that when wet. The wet strength may be regained by soaking the cast in slurry water, which is used to minimize dissolution of the surface during soaking.

Problem 6

A gypsum investment was mixed with water in the proportions recommended by the manufacturer, but the working time was too short to invest the wax pattern. What may have happened, and how can this problem be solved?

Solution a

The powder may have been contaminated by water, from high humidity or a wet dispensing spoon. Investment should be stored in an airtight and waterproof container.

Solution b

The temperature of the mixing water may have been higher than 23" C. Water at 20' to 23" C is recommended. Higher temperatures shorten the working time.

Solution c

The spatulation may have been done incorrectly with a mechanical mixer. Overmixing, too long or too rapid, shortens the working time.

Solution d

The mixing bowl or spatula may have been contaminated with particles of set investment containing calcium sulfate dihydrate, accelerating the reaction. Mixing implements should be cleaned before use.

Problem 7

A full crown casting prepared by an immersion hygroscopic technique was too loose. What conditions might have caused this problem, and how can a tighter crown be obtained?

Solution a

The water bath may have been warmer than usual. When warmer water is used, the wax pattern offers less resistance to the expansion of the investment, producing a larger mold. The temperature of the water bath should be monitored regularly.

Solution b

A thicker-than-average mix of investment may have been used, causing an increased setting and hygroscopic expansion. The water/ powder ratio recommended by the manufacturer should be used, and the water and powder accurately dispensed.

Dental Plaster and Stone

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